What is the lowest Delta_P , for capillary tube in refrigeration . Can capillary works under vacuum Condition i.e Condenser Pressure 50 kPa and Evaporator pressure 10 kPa ?
Capillary can work both above and under atmospheric pressure. The maximum/minimum pressure difference between outside and inside of the capilar depends on the capilar radius and thickness.
This depends on the working fluid, refrigerant type you are using and the design operating conditions in the evaporator and condenser. Based on that you use the p-h chart, tabulated data, or REFPROP program to look for the properties and desing capillary diameter and length. In your problem, you have 40 kPa pressure difference, in addition to the refrigerant flow rate you are using, you can obtain the length of the capillary based on the selected diameter. You may be worried about the durability of the capillary to resist the external pressure, atmospheric pressure 100 kPa, which still the difference between external and internal pressure is that not very high. You will have the lowest internal pressure that you care about in your design at the suddenly contraction (tube inlet) that could be below the 10 kPa, then a flow reattachment occurs till the 10 kPa.
I have calculated length by using 1 mm diameter tube for methanol as a refrigerant in ADsorption chiller. The answer of length is negative ( Minus digit). And by changing diameter to 2.3 mm, length is apx. 1 meter. So i am confused. What is interpretation of such answer.
Which equations you used to calculate tube length? Did you use Darcy's eqn? What are the parameters you used to obtain the length? Where did the minus sign come from?
Yes, I used Darcy equation, and for methanol , I took all properties for 65,50,25 and 10 degree Celsius.By keeping vacuum pressure, and assuming 1 mm bore dia of capillary.
In Darcy's eqn. you use pressure differeence, DP, so the length can be obtained by
L = 2*diameter * DP/(f. rho. V2). So make sure that you are using pressure difference, which should be positive.
And don't forget to see if the cappilary can hold the pressure difference. According to the mechanical of material, the stress due the pressure is calculated by: s = p.r/t (normal stress equals to pressure times radius divided by thickness). Use the maximum pressure difference between internal and external environment.
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